The general rule of thumb is you do not recap the cost of repairs or newly installed items on a used car. Perhaps, if you're lucky, you get back 50% in the increased sales price. However, by not having service records or the need of upcoming service (such as cam belt/water pump) the car may be under valued or much more difficult to sell.
Buyers often negotiate with the request that reducing the sales price by the amount that pending service or repairs would cost.
Good luck, I hope your Porsche 928 finds a good home.
Rich
Membership Chairman
Charter Member
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast